Influenza, Respiratory Virus Activity Fell During Height of Pandemic

Many respiratory viruses demonstrated atypical activity levels as compared with previous rates of activity during pre-pandemic seasonality, the CDC said this week.

Influenza and other respiratory virus activity decreased during the COVID-19 pandemic, likely due to the implementation of mitigation practices to fight the transmission of COVID-19 such as mask use, physical distancing, stay-at-home orders, and prohibition of global travel, according to research published in the CDC’s Morbidity and Mortality Weekly Report for the week of July 23, 2021.

Influenza activity decreased in March 2020, remained low during the summer months, and was lower than during any previous influenza season since at least 1997 between October 2, 2020–May 22, 2021. Low rates of influenza virus transmission may exacerbate the upcoming influenza season because of the lack of natural exposure to the virus and subsequent low levels of population immunity, the report said.

Rates of other virus activity in respiratory diseases such as respiratory syncytial virus (RSV), common human coronaviruses (HCoV), parainfluenza viruses (PIV), human metapneumovirus (HMPV), respiratory adenovirus (RAdV), and rhinovirus and enterovirus (RV/EV) were atypical, as well.

The weekly percentage of positive RSV results during January 4–April 4, 2020 decreased from 15.3% to 1.4% and then remained low (<1.0% per week) until April 2021, when RSV activity increased.

The weekly percentage of positive HCoV results during January 4, 2020–April 18, 2020 declined from 7.5% to 1.3% and remained <1.0% until February 27, 2021. Positive test results for the virus increased to 6.6% by May 22, 2021, primarily led by types OC43 and NL63.

The weekly percentage of PIV test results during January 4, 2020–March 28, 2020 decreased from 2.6% to 1.0% then remained <1.0% until April 3, 2021. By March 22, 2021, positive test results increased to 10.9%, primarily led by type PIV3.

The weekly percentage of HMPV results increased from 4.2% to 7.0%, then decreased to historically low rates of 1.9% during the week of April 11, 2020. The rate of positive test results remained <1.0% through May 22, 2021.

Although the dramatic decrease in circulation of HMPV is unexplained, the disruption in transmission of PIVs and HCoVs could be attributed to school closures and online classes implemented until late winter, as the viruses typically circulate at high levels among children.

The weekly percentage of positive RAdV results during January 2020–April 2021 decreased to historically low ranges of 1.2%–2.6%, then increased to 3% by May 22, 2021.

The weekly percentage of positive RV/EV results decreased to 9.9% by January 16, 2021, then increased to 19.1% on May 22, 2021, a rate similar to those of pre-pandemic seasonal levels.

The prolonged survival of nonenveloped viruses RAdV and RV/EV on surfaces may contribute to their increased resistance to mitigation practices like mask wearing and surface cleaning and explain their return to pre-pandemic transmission patterns.

The recent increases in rates of positive test results for certain respiratory viruses could suggest that pre-pandemic seasonality is resuming, and rates may continue to increase as COVID-19 mitigation practices begin to wane. Clinicians should be mindful of heighted respiratory virus activity, regardless of season, the report said.

Practicing everyday preventive action and establishing fall influenza vaccination campaigns are critical to the prevention of respiratory disease, the authors said, as nonpharmaceutical interventions adopted in the pandemic begin to relax and schools and workplaces return to in-person operations.


Olsen SJ, Winn AK, Budd AP, et al. Changes in influenza and other respiratory virus activity during the COVID-19 pandemic—United States, 2020–2021. MMWR Morb Mortal Wkly Rep. 2021;70:29. 1012-1019.

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